Process of moistureproofing



Jams, 1935. y lN'FWEACH 1,987,045

PROCESS OF MOISTUREPROOFING Filed July 16, 1932 Patented Jan. 8, 1935` UNITED STATES PATENT OFFICE PROCESS 0F MOISTU'REPROOFING Application July 16, 1932, Serial No. 622,955

6` Claims.

This invention relates to a process for producing a thin, flexible, transparent type of sheeting adapted for wrapping purposes and more particularly to a process for preparing cellulose ester lsheeting characterized by an extremely high resistance to the transmission of moisture.

A considerable amount of research work has been carried on with the object of finding a means of improving the moisture resisting qualities of cellulosic thin sheet material. Most of this work has been directed toward improvement of the various types of moistureproong compositions with which the sheeting is impregnated or coated. In developing the present invention the problem was attacked from an entirely different angle with the result that an unexpected relation between the residual solvent content of cellulose ester sheeting. and the ability of the coated sheet to transmit moisture was discovered.

I have found that if unmoistureproofed cellulose ester thin sheeting is given a preliminary treatment or is formed under conditions which reduces its solvent content below a certain critical value, the ability of the coated sheet to transmit moisture is very definitely reduced. That the moisture transmission characteristics of untreated sheeting are dependent upon its solvent content will be apparent from the accompanying drawing and the following description.

The principal object of my invention is to provide a process for producing a thin, exible, transparent type of moistureproof cellulose ester sheeting. A further object is to provide a means whereby the moisture transmission characteristics of the nished sheeting may be definitely controlled. A specific object is to provide a process for producing a moistureproof type of thin cellulose acetateI sheeting having a denite limit of solvent in its base. Other objects Will hereinafter appear.

These objects are accomplished by the following invention which comprises subjecting the cellulose ester sheeting to a preliminary drying treatment either during or after its formation whereby its residual solvent content is reduced to or below a critical value which, in the case of v cellulose acetate sheeting containing residual acetone, corresponds to about 3 to 3.5 or 4% by weight of solvent based on the weight of the cellulose acetate itself. The material is then passed directly to a moistureproong operation in which a solution, such as a mixture of gum and wax dissolved in a suitable solvent is applied thereto, solvents are evaporated, and the sheet dried n at a proper temperature.

In the following description, I have set forth several of the preferred embodiments of my invention, but they are included merely for purposes of illustration and not as a limitation thereof.

My process is preferably carried out as follows: A thin, flexible transparent sheet of unmoistureproofed cellulose acetate containing residual solvent to the extent of approximately 6% or 7% is passed upwardly through a chamber about 20 ft. high having a rectangular cross-section of about 8" X 48" through which a current of dry air is passed at the rate of approximately 800 cu. ft. per minute. The temperature of the enteringv air is maintained at about 230 F. and the exit air at at least 180 F., and the speed of passage of the lm vthrough the air chamber is about 10-30 ft. per minute. While it is desirable to expose the sheeting for a length of time sufficient to bring the solvent content down to the desired value, it is my belief that the temperature to which the material is subjected is of more importance in arriving at the desired critical value of residual solvent content than is the duration of the exposure.

It will be apparent that the process described in the foregoing paragraph relates to the treatment of a previously formed sheet, but my invention is in no sense limited thereto as the solvent reduction treatment may comprise one of the steps in a continuous operation in which the cellulose ester sheet is formed. In such a process a suitable cellulose ester solution or dope may be caused to flow onto the coating roll of a conventional type of roll-coating machine, such as that disclosed in the patent to Sulzer, Patent No. 1,466,733, the solvents partially evaporated therefrom, and the sheet stripped from the coating surface and carried through an appropriate drying cabinet of the general nature of that described in the previous paragraph where the residual solvent is brought down to the desired value, thence through an appropriate coating apparatus where the moistureproong composition is applied, followed by further drying, after which the finished sheet is wound on storage rolls and is then ready for use.

The sheeting prepared as described above is moistureproofed by the application on one or both sides of a gum-wax overcoating followed by appropriate drying. The moistureproong may be accomplished in a variety of ways. It may, for example, be carried out in accordance with the procedure outlined in the co-pending application of Norman F. Beach and Bruce E. Gramkee, Serial No. 551,491, filed July 17, 1931, in which a duced below a certain critical value.

solution of gum and wax in a common solvent is applied to the cellulosic base sheet by subbing rolls, or in case of application to both sides of the sheet, by immersion in the solution. In some cases, it may be preferable to employ the procedure outlined in the application of Norman F. Beach and James G. McNa1ly, Serial No. 551,490, filed July 17, 1931, in which the cellulosic sheeting is first coated with a gum or resin solution followed by application of a wax solution.

While I have described my invention by reference to a sheet of cellulose acetate containing acetone, my invention is in no sense limited thereto as it is equally applicable to the production of moistureproof sheeting by pretreating base sheets formed of cellulose propionate, cellulose butyrate, cellulose stearate, cellulose acetate propionate, cellulose acetate butyrate, cellulose acetate stearate, and the like. Likewise, the solvent which is removed according to the principle' of the in vention may be any simple solvent, such as acetone, or a mixture of various appropriate solvents. The residual solvent will, of course, vary with the type of cellulose compound from which the sheet is formed, and the present invention is in no sense limited to the removal of a particular type of solvent from a particular type of cellulose ester material. On the contrary, the process is founded on the broad discovery that there is definite relation between the residual solvent content of the base sheet and its moisture transmitting qualities after moistureproong and is accordingly applicable to any cellulose ester sheeting.

The accompanying curve, plotted from experimental data, illustrates graphically the remarkablevchange in the moisture transmission characteristics of a moistureproof cellulose ester sheet occurring when the solvent content has been re- It will be noted that the curve begins to slope sharply at a point corresponding approximately to a solvent content, in the case of acetone, of about 5.5% and again changes its slope rather markedly in the vicinity of about 3.5% acetone, thus indicating that the most satisfactory moisture transmission is obtained when the range of solvent content is below about 5% and preferably below 4% or even about 3%.

Several methods of determining the moisture transmission of a moistureproof cellulosic sheet may be employed. A convenient and accurate method may be carried out as ffollows: A weighing bottle'containing a weighed amount of a Water absorbing agent (magnesium perchlorate tri-hydrate) is placed in a crystallizing dish and thereafter a diaphragm made up from the sheeting undergoing test is cemented over the mouth of the dish. 'Ihe dish is then placed in an atmosphere saturated with water vapor at a temperature between and 95 F. After a convenient length of time, the diaphragm is removed and the weighing bottle transferred to a balance and weighed. The permeability, or gain in moisture, is expressed in milligrams per square centimeter per hour.

As many apparently widely different embodiments of this invention may be made without departing from the spirit thereof, it is to be understood that I do not limit myself to the foregomoistureproof cellulose esterwrapping sheetwhich comprises coating a cellulose ester solution upon an appropriate surface, partially evaporating solvents from the resulting sheet, stripping the sheet from `the coating surface, reducing its solvent content to at least 5% and thereafter applying a moistureproong composition thereto.

3. The continuous method of making a thin moistureproof cellulose acetate wrapping sheet which comprises Icoating a cellulose ester solution upon an appropriate surface, partially evaporating solvents therefrom, stripping the resulting sheet from the surface and passing it through a drying chamber to reduce its solvent content to at least 5%, and thereafter applying a gum-wax moistureproong composition thereto. 4. A moistureproofing wrapping sheet `comprising a thin, flexible sheet of cellulose acetate having a moistureproof coating thereon and containing not more than about 5% residual solvent. y

5. A moistureproof wrapping sheet comprising a thin, flexible sheet of cellulose acetate having a gum-wax moistureproof coating thereon and containing not more than about 5% of residual p acetone. 

